This invention relates to electron beam deflection control apparatus for use in a beam index color television cathode ray tube and, more particularly, to such apparatus which achieves desirable uniformity in the scanning speed of the electron beam so as to avoid non-linearities therein.
Beam index color television receivers have long been known, such as described in U.S. Pat. No. 2,791,626 to Hergenrother; U.S. Pat. No. 2,945,087 to Graham et al.; U.S. Pat. No. 3,042,392 to Keiper et al.; U.S. Pat. No. 3,249,688 to Davidse et al.; and U.S. Pat. No.4,003,082 to Fumoto. In a typical beam index color television receiver, the cathode ray tube (CRT) has a single electron beam which scans the color phosphor elements provided on the display screen of the CRT, the scanning electron beam being modulated with the appropriate color information as the beam scans the respective color elements. This differs from the conventional color CRT in which three individual electron beams simultaneously scan the color phosphor stripes with each beam being modulated with its associated color information.
A typical display screen of a beam index CRT is described in copending application Ser. No. 969,975, filed Dec. 15, 1978. The color phosphor stripes, which are formed as triads of red (R), green (G) and blue (B) stripes, are arranged sequentially and in the horizontal scanning direction so that the beam repeatedly scans RGB RGB . . . phosphor stripes. When the beam scans the R stripe, red color information is used to modulate the intensity of the beam, and when the beam scans the G stripe, green color information is used to modulate the intensity of the beam, and when the beam scans the B stripe, blue color information is used to modulate the intensity of that beam. An index signal is generated in synchronism with the scanning of the electron beam so as to control the "switching" of the color information at the appropriate times so as to modulate the beam with the red, green and blue information as the beam scans the R, G and B phosphor stripes, respectively.
As described in the aforementioned copending application, the index signal is derived from a series of fluorescent or phosphorescent index stripes which are arranged behind the color phosphor stripes and in predetermined positional relationship therewith. As the beam scans its raster, the beam first impinges upon the index stripes and then upon the color phosphor stripes. This impinging electron beam excites each index stripe, in sequence, to emit light which is detected by a photodetector disposed so as to receive the light emitted by each excited index stripe. Consequently, the photodetector generates an index signal whose frequency is determined by the frequency of the light emitted by the scanned index stripes.
Of course, the frequency of the index signal derived from the photodetector is a function of the pitch of the index stripes and the horizontal scanning speed of the electron beam. As the speed of the beam increases, the index frequency, that is, the frequency of the index signal, likewise increases. If the pitch of the index stripes is increased (or decreased), depending upon the particular size of the CRT display screen, the technique used to place the index stripes on the screen, etc., the index frequency is reduced (or increased). Hence, if the electron beam scans in a non-linear manner, the non-uniformity in the scanning speed thereof introduces corresponding non-uniformities in the index frequency.
As is known, there is a time delay in the television signal processing circuit of the beam index color television receiver from the time that the photodetector detects the index signal to the time that the "color switching" is carried out. That is, the position of the electron beam will be advanced at the time that the intensity of that beam first is modulated with respective color information. This time delay generally is constant and, therefore, may be compensated so as to assure that red information is switched to modulate the intensity of the beam at the appropriate time that the beam reaches the R phosphor stripe, that the green information is switched to modulate the intensity of the beam at the appropriate time that the beam reaches the G phosphor stripe, and the blue information is switched to modulate the intensity of the beam at the appropriate time that the beam reaches the B phosphor stripe. However, if the horizontal deflection linearity of that beam varies from its normal level, that is, if the scanning speed of the beam is increased or decreased, the resulting change in the index frequency produces a phase shift in the times at which the red, green and blue information signals are switched to modulate the beam intensity. As a consequence of this disturbance in the timing of the color switching, color misregistration occurs. Thus, there is a definite need in detecting non-linearities in the scanning speed of the electron beam in a beam index color television receiver, and to compensate for such non-linearities in order to attain proper color registration of the displayed video picture.
Although the prior art is aware of various techniques and systems which have been used in so-called conventional color television receivers to improve the linearity of the horizontal deflection of the electron beams therein, such prior art techniques are not successful in overcoming the problem of color misregistration due to such non-linearities in a beam index color television receiver.